A reliable operation of an electric power system supplying electricity is expected by all users thereof, e.g. homes and industries. In order to provide the requested reliable operation, apparatuses of the electric power system need to be fault-tolerant and enable continued operation even in the event of failure of components of the apparatus.
As a particular example of such apparatus, a static compensator (STATCOM) can be mentioned. The STATCOM comprises a voltage source converter (VSC) having an AC side connected to an AC network and a DC side connected to an electric power storage such as capacitors. The voltage output from the STATCOM AC side is controlled and the STATCOM thereby acts to supply or absorb reactive power from the AC network. The STATCOM can thereby offer voltage stability to the electric power system.
One type of VSC comprises a chain-link converter, also denoted cascaded H-bridge converter. In such VSC a number of cell modules are series-connected. It is readily understood that when increasing the number of cell modules (number of power semiconductors), the risk of failure increases.
The VSCs, being an apparatus of the electrical power system, need to be fault-tolerant, thus enabling continued operation thereof even in the event of a failure. Such fault tolerance can be achieved in different ways. One way is to provide redundancy, e.g. by bypassing a cell module containing the failed device and continue operation with the undamaged cell modules.
WO 2010/102666 discloses methods and means for a failure mode of operation of a voltage source chain-link converter. The voltage source chain-link converter described comprises a number of converter cell modules connected in series, and one or more redundant converter cell modules are provided. Each converter cell module comprises two phase legs. In order to bypass a faulty converter cell module, the phase leg comprising the faulty converter cell module is blocked and the other phase leg is driven to provide zero AC voltage.
The devices and methods of WO 2010/102666 are well-functioning, but it would be desirable to provide alternative means and methods for providing fault-tolerance of such a voltage source chain-link converter.